Thermodynamic and Physical Properties of Molten Sodium Polysulfides from Open-Circuit Voltage Measurements

Abstract

The open‐circuit voltage of the cell /pyrolytic graphite has been measured as a function of temperature and sodium‐polysulfide composition. From these data, differential free energy, entropy, and enthalpy values for the potential‐determining reaction and the differential free energy of formation for various polysulfide compositions were calculated. The partial differential enthalpy of adding Na to the polysulfide melt is −47.6 kcal/mol independent of melt composition and temperature. The solubility of sulfur in sodium pentasulfide was determined between 280°–390°C, and the phase diagram of Pearson and Robinson was revised in the range and extended to show the Na‐rich border of the miscibility gap.